During many medical procedures, various fluids are injected into patients for purposes of diagnosis or treatment. An example of one such fluid is contrast media used to enhance angiography or CT imaging. The injectors used in these procedures are automated devices that expel the fluid from a syringe, through a tube, and into the subject.
Injectors suitable for these applications must have relatively large volume syringes and be capable of producing relatively large flow rates and injection pressures. For this reason, injectors for such applications typically include large, high mass injector motors and drive t rains. These are typically housed in an injection head, which is supported by a floor, wall, or ceiling mounted arm.
In many such injectors the syringes are replaceable. In one exemplary injector, to replace the syringe, a front face plate which facilitates coupling between the syringe plunger and the plunger drive ram is moved, the used syringe detached, and a fresh syringe attached.
When a used syringe is removed, a replacement syringe is installed on the injector. The rearward face of the syringe plunger has a coupling element referred to as a button. This button is mushroom-shaped and consists of an extension protruding from the rearward face of the syringe plunger, topped by a cap. When the new syringe plunger is inserted, the button contacts the plunger coupling engagement jaws located on the forward end of the plunger drive ram. These jaws then snap around the button of the syringe plunger, thereby connecting the syringe plunger to the plunger drive ram.
This "jaw and button" coupling mechanism in an injector does not result in a perfect fit between the jaws on the plunger drive ram and the button on the rearward face of the syringe plunger. The ends of the jaws are not completely snug around the cap of the button on the rearward face of the syringe plunger. This results in some amount of "backlash" between the mating sections of the coupling mechanisms. Due to this spacing the syringe plunger does not move cooperatively with the plunger drive ram upon motorized, automated movement of the plunger drive ram. Rather, the jaw and button embodiment features an initial movement of the plunger drive ram and a subsequent responsive movement of the syringe plunger. Thus, there is an element of lag time, albeit small, in deploying the syringe plunger to inject fluids to a subject. This is an unfortunate problem because many medical applications require an extremely high degree of accuracy, especially in the timing and volume of substances injected into the body. The lag time inherent in the jaw and button arrangement creates a reduced degree of accuracy.
Accordingly, to improve accuracy, there is a need for a coupling mechanism for the syringe plunger and plunger drive ram where movement of the syringe plunger is directly cooperative with movement of the plunger drive ram, allowing for enhanced accuracy in the volume of a fluid injected into a subject.